The present invention relates to registration of plural image exposures formed on a photoreceptor belt by a plurality of Raster Output Scanning (ROS) systems and, more particularly, to a method and apparatus for forming registered color images on the belt in a single pass.
In single pass electrophotographic printers having more than one process station which provide sequential images to form a composite image, critical control of the registration of each of the sequenced images is required. Failure to achieve registration of the images yields printed copies in which the images are misaligned. This condition is generally obvious upon viewing of the copy, as such copies usually exhibit fuzzy color separations, bleeding and/or other errors which make such copies unsuitable for intended uses. A simple, relatively inexpensive, and accurate approach to register latent images superposed in such printing systems has been a goal in the design, manufacture and use of electrophotographic printers. This need has been particularly recognized in the color and highlight color portion of electrophotography. The need to provide accurate and inexpensive registration has become more acute, as the demand for high quality, relatively inexpensive color images has increased.
The present invention is directed towards a single pass, color xerographic system, in which a plurality of ROS stations are positioned adjacent to a photoreceptor surface and selectively energized to create successive image exposures, one for each of the three basic colors. A fourth ROS station is added if black images are to be created as well. Thus, each image area on the photoreceptor surface must make at least three passes relative to the transverse scan lines formed by a modulated laser beam generated by the ROS system. Each image is formed by a series of horizontal scan lines with each line having a certain number of pixels per inch. Each image must be registered to within a 0.1 mm circle or within a tolerance of .+-.0.05 mm. Each color image must be registered in both the photoreceptor process direction (process registration) and in the direction parallel to the process direction (referred to as the fast scan or transverse registration).
The present invention is directed towards a method and apparatus for determining the position of the ROS scanning beam relative to the photoreceptor in the process direction. The relative location is measured using a pair of transversely aligned belt holes, each hole having an associated light detector. As the belt advances into the path of the transversely swept ROS beam, a series of measurements are made as the ROS beam crosses the leading edge of the hole. By collecting the successive signal outputs of the detector, an extrapolation is made which identifies, with a very high degree of accuracy, the exact time that the midpoint of the gaussian-shaped ROS beam is at the lead edge of the belt hole. Process and skew direction adjustments can then be made to each ROS.
More particularly, the present invention is directed towards an electrophotographic printer of the type having a photoreceptor surface mounted for movement in the process direction, the printer including:
a plurality of imagers for sequentially scanning and exposing image areas with a scanning beam to form a composite color image thereon,
a photoreceptor belt mounted for movement in the process direction, said belt having formed therein at least one belt hole having a leading edge which advances with the belt in the process direction,
detecting means associated with each imager for generating a series of increasing output signals up to a maximum, which correspond to the maximum illumination distribution of the scan beam, each signal produced by a separate scan beam crossing the leading edge of the hole, and
circuit means for interpolating the detector output signals to determine the time that the ROS beam is at midpoint exposure level at the hole leading edge and for generating a line sync signal based on this reference time.